This invention relates to plastic compositions containing additives which render the composition degradable by ultraviolet radiation.
The advent of plastics has given rise to improved methods of packaging goods. For example, polyethylene and polypropylene plastic films, bags, bottles, styrofoam cups and blister packages have the advantages of being chemically resistant, relatively unbreakable, light in weight and translucent or transparent. The increasing use of plastics in packaging applications has created a serious waste disposal problem. Burning of these plastic materials is expensive and may add to air pollution problems.
Unlike some other packaging materials, such as paper and cardboard, plastics are not readily destroyed by the elements of nature. Thus, burying them is not an effective means of disposal and can be expensive.
Plastics are biologically recent developments, and hence are not easily degradable by micro-organisms which attack most other forms of organic matter and return them to the biological life cycle. It has been estimated that it may take millions of years for organisms to evolve which are capable of performing this function. In the meantime, plastic containers and packaging films are beginning to litter the countryside after being discarded by careless individuals.
One means suggested for combating the plastic waste disposal problem has been the use of plastic compositions which degrade upon exposure to ultraviolet light. This approach would take advantage of the natural tendency of most organic polymers to undergo gradual reaction with atmospheric oxygen, particularly in the presence of light. Typically, a photosensitizing additive is employed in order to accentuate this natural tendency. The photosensitizing additive absorbs ultraviolet light (e.g., from sunlight); the additive, in the resulting photo-excited state, undergoes a chemical reaction which leads to the generation of free radicals which initiate an autoxidation process thereby leading to the eventual disintegration of the plastic material.
In the course of our investigations of the effect of sensitizing additives on the photodegradation of common thermoplastics, we have discovered that organic photosensitizers such as aromatic ketones and anthraquinone derivatives are quite effective in promoting rapid initiation of photo-oxidative degradation. However, as the reaction proceeds, the rate tends to decrease, presumably as the result of the chemical conversion of the sensitizer to a less effective compound or mixture of compounds.
Organic derivatives of transition metals, such as transition metal acetylacetonates and transition-metal salts of fatty acids, also function as effective accelerators of the photo-oxidative degradation of plastics. However, the utility of such materials as degradation-promoting additives is limited, owing to a tendency to cause thermal oxidation of plastics during high-temperature processing (e.g., fabrication of shaped articles by molding or extrusion); the coloration imparted to plastics by such additives; and the potential toxicity of such additives. Consequently, such additives must be employed in very low concentrations, if at all.
In contrast to other organic derivatives of transition metals, the dialkyldithiocarbamates such as ferric dimethyldithiocarbamate tend to act not only as sensitizers of photo-oxidative degradation, but also as retarders of thermal-oxidative degradation. When employed at low concentrations, such additives also display an "induction period" in photo-oxidative degradation; that is, degradation starts only after a certain delay, during which no appreciable degradation takes place. Once the degradation has started, the process is auto-accelerating, i.e., the degradation rate increases as exposure time increases. While the "induction period" feature is sometimes advantageous in the sense that undesired degradation due to accidental short-term exposure to light is avoided, it is disadvantageous in other instances, e.g., in cases where the lifetime of a discarded plastic article must be kept to a minimum or in cases where plastic wastes receive only a minimal exposure to light prior to earth burial, i.e., as in a land fill.
The present invention provides a route to degradable plastic compositions which display both the rapid initiation of photo-oxidation characteristic of compositions containing organic photosensitizers and the continuing, auto-accelearating degradation process which is characteristic of compositions containing organic derivatives of transition metals. In the practice of this invention a composition containing both an organic photosensitizer and an organic derivative of a transition metal is employed; the organic photosensitizer causes rapid initiation of the degradation process whereas the use of the transition-metal compound provides for a rapid, continuing photo-oxidation process, which is typically auto-accelerating. Furthermore, selected compositions display a "synergistic effect", in that the degradation-promoting effect of the combination of additives is greater than that which would be expected on the basis of the effect of each additive when used alone.
The practice of the present invention also provides a route to compositions which gradually oxidize in the dark after an initial short-term exposure to ultraviolet light. We believe that this unexpected but highly desirable result is due to the rapid build-up of hydroperoxide groups during the photochemical initiation phase, followed by a slower thermal oxidation process caused by metal-catalyzed decomposition of these hydroperoxide groups.